1. Field of the Invention
This invention generally addresses the cross directional control of a process, such as a paper manufacturing process. The invention can improve the accuracy of a stationary array sensor employed for cross directional control by fusing the sensor output with the output of a scanning gauge, or sensor, using a bank of filters such as Kalman filters, where each filter in the bank has a simple computational structure.
2. Background Information
Cross directional (CD) control of processes, such as paper manufacturing processes, is known. For example, U.S. Pat. No. 4,903,528 entitled “System and Process For Detecting Properties Of Traveling Sheets In Cross Direction”, U.S. Pat. No. 4,965,736 entitled “Cross-Directional Control Of Sheetmaking Systems” and U.S. Pat. No. 5,121,332 entitled “Control System For Sheetmaking”, the disclosures of which are all hereby incorporated by reference, are directed to cross directional control using a scanning sensor. A document entitled “Estimation of Cross-Directional Properties: Scanning vs. Stationary Sensors”, Tyler, Matthew L. et al., AIChE Journal, Vol. 41, No. 4, April 1995, pages 846-854 also discusses cross-directional control of a process using a scanning sensor. The scanning sensor is used to perform process measurements in the cross direction of a moving sheet of paper. The measurements serve as feedback for control over some property, such as basis weight, moisture content or coating thickness, to render the property uniform across the moving sheet of paper.
Because scanning sensors move back and forth across the paper sheet as it moves in a machine direction (MD), cross directional variations are not measured directly. Cross directional variations in a property to be measured can occur at a non-negligible rate relative to the speed of MD paper movement and the scan rate of the scanning sensor. Because the profile data associated with a scanning sensor is available only once per scan cycle, the scanning sensor's scan rate can be too slow to adequately address the dynamics of cross directional variations of the process property being controlled.
Newer technologies have been proposed to provide faster sensing of the cross-directional properties of a product, such as the paper sheet in a paper manufacturing process, by using a stationary array sensor. A stationary array sensor includes a plurality of sensors located adjacent to one another in the cross direction, each sensor providing an approximately instantaneous measurement of a given property of the paper at a location across the paper's width. Although stationary array sensors avoid the need of a cross directional scan using motion of a single sensor back and forth over the moving paper sheet, the requirement that the stationary array sensor includes a plurality of individual sensors can render it quite expensive. For example, a stationary array sensor associated with a paper manufacturing process can require on the order of three hundred sensors to cover a sufficient width of the paper. This need for a large number of individual sensors in the stationary sensor array renders it cost prohibitive to include sensors having a high degree of precision. That is, to provide a stationary sensor array that can achieve measurements with an accuracy that approaches the accuracy of measurements achieved with a scanning sensor, expensive individual sensors must be used.
It is also desirable to use stationary sensor arrays at locations in a manufacturing process which are unsuitable for scanning sensors. Again, in these circumstances, the stationary sensor array is typically configured with sensors that render the array cost competitive with scanning sensors used at other locations in the process. For example, stationary sensors are used in the earlier stages of a paper manufacturing process where the presence of flying debris, such as warm paper pulp, could jam the scanning mechanism of a scanning sensor. To achieve maximum benefit from the fast stationary array sensor measurement, such sensors would be used in close proximity to the actuators. Scanning sensors are commonly used in the end of the process, downstream from the actuator such that they measure properties of the finished product for quality control purposes.
In addition to the use of less expensive, less accurate sensors in stationary sensor arrays, another factor which detracts from the quality of the measurements they provide is their susceptibility to drift. Although the sensors of stationary sensor arrays and scanning sensors can both experience drift, it is relatively easy to recalibrate a scanning sensor at least once during each scanning cycle. For example, the scanning sensor can be recalibrated during each cycle by moving it to a location off the paper being produced. Such a recalibration cannot be easily achieved with stationary sensor arrays, wherein each of the sensors is fixed in position.
Thus, processes such as paper manufacturing processes and coating processes which involve cross-directional control, are known which use both stationary sensor arrays and scanning sensors at various locations in the paper production process. It would be desirable to improve the accuracy of a stationary array sensor such that the quality of the measurement provided thereby is comparable to or exceeds that of a scanning sensor without rendering the stationary sensor array substantially more costly than a typical scanning sensor,